In CCD (Charge-Coupled Device) solid-state image sensors, signal charges are generated in an n-type semiconductor region formed at the surface of a p-type silicon substrate, when light is incident on the n-type semiconductor region. A video signal can be obtained based on the signal charges generated in each pixel.
FIG. 16 is a cross-sectional view showing an example of solid-state image sensors as technical background. FIG. 16 shows a region including one of pixels arranged on a p-type silicon substrate 101. In FIG. 16, transfer electrodes 106 are formed on the p-type silicon substrate 101 with an insulating film 105 interposed therebetween. The insulating film 105 is a silicon oxide film. A photoelectric conversion region 102, which is an n-type semiconductor region, is formed in a surface part of the p-type silicon substrate 101 between adjacent two transfer electrodes 106. A light-shielding film 107, which is made of aluminum or tungsten, covers the transfer electrodes 106 with another insulating film 110 interposed therebetween. The light-shielding film 107 has an opening above the photoelectric conversion region 102. A passivation film 108 is formed so as to cover the insulating film 105 located on the photoelectric conversion region 102, and the light-shielding film 107.
Transfer regions 104, which are n-type semiconductor regions, are formed in an upper part of the p-type silicon substrate 101 under the transfer electrodes 106, respectively. Two transfer regions 104 are thus formed with the photoelectric conversion region 102 interposed therebetween. There is a gap between the photoelectric conversion region 102 and one of the transfer regions 104, and this gap serves as a transfer region 101a for transferring charges from the photoelectric conversion region 102 to the transfer region 104. A p+-type semiconductor region 103 for isolating pixels from each other is formed between the photoelectric conversion region 102 and the other transfer region 104. This technique is hereinafter referred to as the first related technique.
The structure of the first related technique shown in FIG. 16, however, has large losses of incident light due to reflection at the surface of the p-type silicon substrate 101, and therefore does not have sufficient sensitivity. Regarding this problem, Japanese Published Patent Application No. H04-206571 (Document 1), for example, proposes to form an antireflection film over photoelectric conversion regions. Hereinafter, a technique represented by Document 1 is referred to as the second related technique.
The second related technique will now be described with reference to FIG. 17. Note that, in FIG. 17, components corresponding to those of FIG. 16 are denoted with the same reference numerals and characters as those of FIG. 16.
In the second related technique, a photoelectric conversion region 102 and transfer regions 104 are formed at the surface of, for example, a p-type silicon substrate 101. The photoelectric conversion region 102 is a region for obtaining signal charges, and the transfer regions 104 are regions for transferring the signal charges read from the photoelectric conversion region 102. The photoelectric conversion region 102 and the transfer regions 104 are n-type semiconductor regions, and are formed by, for example, impurity diffusion. Note that pixels are isolated from each other by a p+-type semiconductor region 103.
An insulating film 105 is formed so as to cover the p-type silicon substrate 101, and an antireflection film 109 is formed on the insulating film 105. The insulating film 105 is a silicon oxide film, and the antireflection film 109 is a silicon nitride film having a refractive index higher than that of a silicon oxide film and lower than that of silicon. A silicon oxide film has a refractive index of about 1.45, a silicon nitride film has a refractive index of about 2.0, and silicon has a refractive index of about 4.1. Each of the insulating film 105 and the antireflection film 109 has a thickness of about 60 nm or less. Preferably, the respective thicknesses of the insulating film 105 and the antireflection film 109 are set to desirable values in the range of about 25 nm to about 50 nm.
By setting the respective thicknesses of the insulating film 105 and the antireflection film 109 to appropriate values in this manner, the reflectance can be suppressed to about 12% to about 13% on average in the visible wavelength region. This means that the reflectance is reduced to about one third of the reflectance in the case of a p-type silicon substrate (about 40%). This achieves high sensitivity.
This structure, however, has the following problem.
As described in, for example, Japanese Patent Laid-Open Publication No. H06-209100 (Document 2), one of effective methods for reducing a dark current in a solid-state image sensor is to diffuse hydrogen to a silicon interface (in the above example, the interface between the photoelectric conversion region 102 and the insulating film 105). When hydrogen terminates dangling bonds generated at the silicon interface, intermediate energy levels decrease, whereby a dark current is reduced. In the structure of the second related technique, however, dangling bonds cannot be sufficiently terminated by hydrogen in a final sintering process.
The reason for this is as follows: if the antireflection film 109 is a silicon nitride film and a hydrogen sintering process is performed after formation of the antireflection film 109, the silicon nitride film (the antireflection film) prevents hydrogen from reaching the silicon interface. Note that Document 2 does not disclose a method for reducing a dark current in a solid-state image sensor having an antireflection film.
Regarding this problem, Japanese Patent Laid-Open Patent Publication No. 2000-12817 (Document 3), for example, proposes to ensure a hydrogen path to a silicon interface. More specifically, a part of an antireflection film is removed above each transfer electrode so that the antireflection film does not prevent hydrogen from reaching the silicon interface even if a hydrogen sintering process is performed after formation of the antireflection film.
Hereinafter, a technique represented by Document 3 is referred to as the third related technique and will be described with reference to FIG. 18. Note that, in FIG. 18, components corresponding to those of FIG. 17 are denoted with the same reference numerals and characters as those of FIG. 17.
In a solid-state image sensor of the third related technique, a plurality of photoelectric conversion regions 102 and a plurality of transfer regions 104 for transferring charges read from the respective photoelectric conversion regions 102 are formed in an upper part of a p-type silicon substrate 101. Transfer electrodes 106 are respectively formed above the transfer regions.
The transfer electrodes 106 are formed on the respective transfer regions 104 with an insulating film 111 interposed therebetween. The insulating film 111 has an ONO (Oxide Nitride Oxide) structure formed by a silicon oxide film 111a, a silicon nitride film 111b, and a silicon oxide film 111c. 
A first insulating film 112, an antireflection film 113, and a second insulating film 114 are sequentially formed over the photoelectric conversion regions 102. More specifically, the first insulating film 112 is formed on the photoelectric conversion regions 102, the antireflection film 113 covers the first insulating film 112, and the second insulating film 114 covers the antireflection film 113. The first insulating film 112, the antireflection film 113, and the second insulating film 114 extend also over the transfer electrodes 106, and a light-shielding film 107 is formed so as to cover the second insulating film 114.
The third related technique is characterized in that the antireflection film 113 has an opening 113a above each transfer electrode 106. These openings 113a enable hydrogen to reach a silicon interface to terminate dangling bonds on the silicon surface even if a hydrogen sintering process is performed after formation of the antireflection film 113. Document 3 argues that a dark current can thus be reduced.
Note that a passivation film 108, a planarizing film 117, and lenses 115 are formed over the light-shielding film 107.